In the hydraulic field, filters are used to clean the hydraulic fluid from various contaminants occurring within a system. When the filter is plugged with much of this contamination that it can no longer function to provide clean fluid and must be cleaned or changed, a pressure differential indicators is used to make that determination.
In most cases, the pressure differential indicator uses a spring-loaded piston that is placed in the hydraulic line with one end connected upstream of the filter and the other end downstream. When the pressure is great enough to compress the spring associated with the piston, the piston will then activate the indicator to show that the pressure differential across the filtering system is sufficient to indicate a clean/replace condition has been reached.
Typical of this type of device is U.S. Pat. No. 4,480,160, issued to Stifelman on Oct. 30, 1984. In this device, a conductive path is formed within the fluid of a single cavity that is used to indicate the clean/change condition. However, electrically conductive contamination can result in a false positive. This device has open contacts, which are exposed to the free flow of fluid, in and out of the sensor. Thus, the risk of failure due to contamination is present. Further, this device uses a glass to metal hermetic seal to insulate the electrically conductive pin from the grounded body of the device. This type of seal is costly and labor intensive which increases substantially the cost of the unit. Finally, this device impacts the downstream (clean) fluid flow in the hydraulic filtering system.
Another device, again typical for this type of indicator, is disclosed in U.S. Pat. No. 7,308,829, issued to Beard et al on Dec. 19, 2007. This device is similar in construction to the one disclosed in the '160 patent, that is, it also has a spring-loaded piston. As found in the '160 patent, open contacts; which are exposed to conductive contamination false positives, are present. This device relies on an air gap or potting to insulate an electrically conductive pin from the grounded body. The air gap puts the sensor at a significant risk for having a short circuit. The potted version has the potential for the epoxy used for potting to wick into the contact area and render the indicator ineffective. Also, with the intricacies of multiple compartments with multiple electrical contacts, the cost of manufacture is unnecessarily high. This device also impacts the downstream (clean) fluid flow in the filtering system.
There is not found in the prior art a pressure differential indicator using a spring-loaded piston method that eliminates the deficiencies noted above and that can be used in a normally closed or open position to indicate the need to change a filter yet is easy and inexpensively manufactured.